But the most remarkable feature presented by the stomach of the Crustaceans is the very complex masticatory apparatus it contains. This consists of a considerable number of pieces, the form and disposition of which vary, and are always singularly in harmony with the kind of food taken and the general habits of these animals. The apparatus, as well from the important office it fulfils, as from its being no where else encountered in so perfect a state of development, were worthy of a description which would swell this article to too large a size; we shall therefore be brief, and merely state generally that it consists of a great number of pieces, so connected as to constitute a kind of solid frame armed in ternally with tubercles or sharper teeth situated around the pylorus, and capable of being moved so as to bruise or tear in pieces the alimentary matters .subjected to their action, and as they are about to pass through this opening.* The different pieces composing this appa ratus vary considerably in the different genera, and even in the several species of the same genus. Still every one of them may be de monstrated with a little care, in the whole of the Brachyura and of the Macroura. They are less numerous, and are singularly modified in proportion as we recede from these types. In the Squilla mere vestiges only of the ap paratus are found in two semicorneous pieces covered with rounded projections ; and its functions are performed by a branch of each mandible which penetrates even to the pyloric orifice of the stomach.
The intestine extends from the pylorus to the anus without curve or convolution in its course (fig.416, d,h). In the superior Crustacea it may be distinguished into two portions, one of which may be named the duodenum, the other the rectum. These two portions where they occur vary extremely both in their nature and in their relative lengths. Sometimes they are separated by a valve (Lobster) corresponding internally to a circular external elevation ; but still more frequently their respective limits are not obviously marked, and among the whole of the inferior members of the family the intestinal canal is entirely cylindrical, and per fectly identical in its constitution through its whole length. The anus is constantly seated in the last ring, and is closed by certain mus cular fibres which perform the office of a sphincter.
The biliary apparatus of the Crustacea is of very large size in the Decapoda. The liver is symmetrical (fig. 417), and consists of two halves generally separate one from another, and the whole organ is made up of an agglo meration of ccecums, which by one of their extremities empty themselves into excretory ducts. These by their union form larger and larger trunks, and the secreted fluid or bile is finally poured by a double channel into the pyloric portion of the stomach. The liver is
found to undergo extensive modifications as it is examined in individuals lower and lower in the series; in the Edriophthalmians, finally, we discover nothing except three pairs of bili ary vessels analogous to those of insects.
The liver is not the only secerning organ whose product is poured into the intestine. On each side of the pyloric portion of the stomach, we observe two blind tubular cavi ties narrow and much elongated in their form, which pour out a whitish fluid (fig. 416,f,f); and at the point of conjunction of the two por tions of which the intestine frequently consists, as has been said, there is a third tubular cavity or vessel in all respects similar to these two (fig. 416, g). These tubuli are all wanting in the Astacus fluviatilis, and in the Astacus ma rinus the single posterior tubulus is the only one found. Nothing positive is known with regard to the uses of the fluid secreted in these tubu 1 i .
To conclude, there are two organs of a green colour situated on either side of the cesopha gus, the structure of which is glandular, and which appear to bear some analogy to the sali vary glands.
B. Of the blood and are altogether without positive information as to the mode in which the nutritious fluid, elabo rated by the process of digestion, passes from the intestinal canal into the torrent of the cir culation. Hitherto no chyliferous vessels have been detected, and we are therefore led to believe that it is by imbibition that the trans ference takes place from the intestine to the bloodvessels in the Crustacea.
The blood of the Crustacea is a colourless, or slightly bluish coloured fluid, holding an abundance of circular-shaped globules in sus pension. It is extremely coagulable. Its che mical composition has not been investigated.
This nutritious fluid is put into motion by a heart, and circulates through a vascular system of great complexness. Willis,* Swarnmer dam,t Cuvier,t Desmarest,§ and several others have given a description of this system ; but there are still innumerable points upon which opinions remain different. The following are the conclusions to which M. Audouin and I have come from a careful study as well of the anatomical disposition of the circulatory apparatus of the Crustacea, as of the progress of the blood through its interior.11 The circulation of the blood in these ani mals is accomplished in a manner very similar to what takes place in the Mollusca. The blood pushed forward by the heart is distri buted to every part of the body, from whence it is returned into large sinuses situated at no great distance from the base of the branchim; from these sinuses it is sent on to the respi ratory apparatus which it traverses, and from which it then finds its way to the heart, to recommence the same circle anew. The heart is consequently aortic and single.